On verbs

I'm going to buy a book.

Vou comprar um livro. (normal)
"Going (to) buy a book".

It is not necessary to use "I" because the subject is implicit in the verb "to go". Verbs in portuguese express the subject in the termination.

Eu vou comprar um livro. (emphasizing "I")
"I (am) going (to) buy a book".

I buy
you buy
he/she/it buys
we buy
they buy

Verb remains pretty much unchanged.

eu compro
tu compras
ele compra
nós compramos
Eles compram

Even if we don't have the subject, we know that the subject is either I/you/it/we/they because of the verb termination.